Seawater pre-deaerator process for open-cycle ocean thermal energy conversion applications

ABSTRACT

A seawater deaerator has a large reservoir through which seawater slowly flows. Gas is injected into the bottom of the reservoir through porous aeration stones forming bubble nuclei. The seed bubbles move upward through the seawater in the reservoir expanding but not coalescing, and withdrawing dissolved gas from the seawater. The deaerated seawater flows out of the reservoir and subsequently flows through spouts into a flash evaporator. Gas is withdrawn from a low pressure gas chamber at the top of the reservoir by a vacuum pump. The exhaust of the vacuum pump supplies gas to the air injectors.

This application is a division of application Ser. No. 350,912, filedMay 12, 1989, now U.S. Pat. No. 5,096,544.

BACKGROUND OF THE INVENTION

In about 1859 William Rankine described a Rankine cycle in which wateris vaporized to steam, work is produced by the steam and the steam iscondensed. In about 1909 Georges Claude described the use of a Rankinecycle to produce work with the temperatures and pressures available intropical seawater. One of the problems in producing work from tropicalseawater is that dissolved gases are included in the water. Expandingand compressing the gases requires and wastes energy. Moreover the gasesinterfere with the heat flow characteristics, especially in thecondenser. The present invention is directed to solving problemsassociated with the dissolved gases in seawater. The invention hasspecial application to the removal of gases from water in open-cycleocean thermal energy conversion (OC-OTEC) plants. The invention hasapplication in situations in which it is desirable to deaerate or toremove gases from seawater. The invention also has application in theseparation of gases from liquids.

Seawater contains dissolved gases, primarily nitrogen and oxygen.Dissolved gas concentrations in the cold water layer of the ocean nearthe Hawaiian islands indicate that oxygen is significantly belowsaturation and nitrogen is at or slightly below saturation with respectto air at atmospheric pressure. Carbon dioxide is significantlysupersaturated in this cold water layer. The warm surface water isgenerally slightly super-saturated with both nitrogen and oxygen whilecarbon dioxide is below saturation.

In an open-cycle ocean thermal energy conversion plant, dissolved gaseswill be released in the evaporator and direct contact condenser (ifused). By accumulating near the condensing surface, the inert gaseslower the partial pressure of the steam, hence lowering the saturationtemperature of the steam. Reduction of the temperature differentialbetween the saturation temperature and the temperature of the condensingagent lowers the amount of heat flow in the condenser. Simultaneously,these gases raise the condenser pressure. In order to maintain the lowpressure required present open-cycle thermal energy conversion designconfigurations remove these gases continuously by means of a vacuum pumpattached to the condenser, i.e., at the lowest pressure in the system.

The usual plan for the removal of the non-condensible gases is tocompress them to atmospheric pressures and release them to theatmosphere. This imposes a significant burden on the overall systemefficiency with parasitic losses in the range of 10 to 15% of the grosspower depending on the fraction of non-condensibles which evolve.

The other method for handling the non-condensibles is pre-deaeration.Pre-deaeration occurs upstream of the evaporator or the direct contactcondenser (if used). It requires additional system components. Theadvantage of this deaeration strategy is that the evolved gases areremoved at a higher pressure than the boiling point. In addition,accumulation of non-condensibles near the condensing surfaces does notoccur and the related inefficiency can be avoided.

As noted, post-deaeration method gases are removed from the condenser atthe lowest pressure in the system. Removal at this point requiresgreater compression power than for pre-deaeration. In post-deaeration,however, no additional components are required and additional hydraulichead losses, which are associated with pre-deaeration devices, areavoided.

The deaeration technique presented herein is a pre-deaeration process.

SUMMARY OF THE INVENTION

A seawater deaerator has a large reservoir through which seawater slowlyflows. Gas is injected into the bottom of the reservoir through porousaeration stones forming a bubble nuclei. The seed bubbles move upwardthrough the seawater in the reservoir expanding but not coalescing andwithdrawing dissolved gas from the seawater. The deaerated seawaterflows out of the reservoir and subsequently flows through spouts into aflash evaporator. Gas is withdrawn from a vacuum chamber at the top ofthe reservoir by a vacuum pump. The exhaust of the vacuum pump suppliesgas to the air injectors.

The invention especially works in seawater because of the function ofcoalescence. Because of the particular relations of electric charges,surface tension and viscosities, small bubbles tend not to coalesce andnot to agglomerate into larger bubbles. The existence of smaller bubblesin seawater means larger surface areas with the same volume of gas.Consequently, the smaller bubbles are effective in removing dissolvedgas from the water.

The invention has several applications, for example, in aquaculture. Theinvention may be useful in situations where it is desirable to removegas before propelling fluid to avoid cavitation. When used in seawaterpre-deaerator for open cycle ocean thermal energy conversion plants, thepresent invention provides a 20-fold energy savings over other knowndevices.

Deaeration effectiveness is strongly dependent on the pressure in thedeaerator vessel. With low pressures, the saturation concentration ofdissolved gases is low, which gives rise to a high driving force for gastransfer.

Deaeration rates are low if no bubble seeding occurs in the intakewater. With moderate air injection the degree of deaeration increaseswith the volume of injected air, provided that optimum air injectionconditions prevail.

Deaeration rates in the barometric upcomer alone are insufficient foroptimum open-cycle ocean thermal energy conversion applications. A waterreservoir has to be incorporated in the deaeration system to allow for alonger residence time of water at low pressure.

Injection of air should occur in the low pressure water reservoir andnot in the barometric riser. Air injection into the barometric leg islikely to stimulate undue bubble coalescence.

Air injection should be such as to produce small bubbles. Air injectiondone in a wide spread pattern of appropriate air stone injectors wouldavoid appreciable concentration of injected air and the resultingformation of larger size air slugs.

A preferable source for bubble nuclei injection is the exhaust of thevacuum pumps. This procedure would have the advantage of circulating theair in a low pressure environment and would thereby significantly reducethe power requirements for the vacuum compressor system.

The choice of the optimum depth of the water reservoir depends on twoconflicting criteria: a) The water column has to be deep enough toprovide sufficient residence time for bubble rise. b) The water columnshould not be too deep, since the pressure exerted on bubbles increases,thereby decreasing the deaeration performance of system and requiringhigher pressure at the injector.

The water reservoir is the main component of the recommended deaerationprocess and serves the following functions: a) It allows a longerresidence time for the intake water in a lower pressure environment. b)It allows air injection to take place at low pressure. c) It provides awater body where injected bubbles remain suspended for an adequate timefor deaeration to occur. d) It provides a water body where bubbles canrise to the surface and be separated before the water enters theevaporator or the condenser (if direct contact condensation is used).

The evolution of non-condensible gases in the OC-OTEC evaporator andcondenser may result in a deterioration of the overall performance.

Preliminary tests indicated that gas transfer is more efficient innatural seawater than in fresh water. Subsequent studies have beencarried out to identify the probable reasons and mechanisms for theaccelerated gas transfer processes in seawater. In addition, some basicengineering requirements for pre-deaeration as well as reinjection intothe downcomer were investigated. Experiments have been performed toquantify the mechanisms regulating gas transfer in bubbles and in apacked column.

The results of these experiments suggest fundamental differences in themagnitudes of the overall gas transfer process in seawater and freshwater. Bubble coalescence is significantly less in seawater than infresh water, resulting in greater liquid-gas interfacial area andconsequently higher transfer rates in seawater. However, single bubbleexperiments showed that the magnitude of overall transfer is similar inseawater and in fresh water. The liquid transfer coefficient, whichexpresses gas transfer rates per unit surface area, for seawater isslightly smaller than or equal to that of fresh water. Gas transferrates for dissolved nitrogen were observed to be similar to those fordissolved oxygen.

Gas liberation in OC-OTEC subsystems is primarily dependent on thesystem vacuum, the residence time of the water at low pressure and thetype and magnitude of bubble seeding. Even with bubble seeding the totalamount of deaeration in the barometric upcomer alone is relatively low.Consequently, a reservoir is incorporated in the feed water system toincrease the residence time at low pressure of the water to bedeaerated. An OC-OTEC deaeration device was designed from theexperimental results. Design particulars of the device include a lowpressure reservoir, a seed bubble injection device in the reservoir andgas-liquid separation just upstream of the evaporator and/or directcontact condenser. The seed bubble stream is provided from the vacuumpump discharge. Provided appropriate system conditions are met,approximately 85% of dissolved gases may be removed from the OC-OTECfeed stream. A schematic of the pre-deaeration system for the warm waterstream is given in the accompanying figures. A post-reaeration schemewas developed which uses part of the vacuum pump exhaust to partiallyreaerate the OC-OTEC discharge stream. The use of such a hydrauliccompressor is more efficient than mechanically compressing these gasesfor discharge to the atmosphere. Such a system also eliminates thedischarge of excessive amount of carbon dioxide to the atmosphere aswell as increasing the oxygen content of the discharge water.Preliminary experiments show that reaeration is more efficient inseawater than in fresh water.

Adaptation of these procedures in the design of OC-OTEC systems resultsin increased efficiency, decreased cost and the elimination of somepotentially damaging environmental impacts.

An object of the invention is the provision of a seawater deaeratorhaving a water reservoir, a vacuum chamber above the water reservoir.Conduction means moves gas to the vacuum chamber from the waterreservoir to conduct gas into the vacuum chamber. A vacuum pumpconnected to the vacuum chamber evacuates the vacuum chamber. Gasinjectors near a bottom of the water reservoir inject gas in the waterreservoir. A conduit connected to an exhaust of the vacuum pump suppliesexhaust gas from the vacuum pump to the gas injectors. The air injectorsrelease gas into the reservoir in fine seed bubble nuclei.

Exhaust spouts supply water from the reservoir to a flash evaporator. Awater intake supplies water to the reservoir. Water is directed from theintake through the reservoir away from the exhaust spouts.

The preferred exhaust pump reduces pressure in the vacuum chamber toabout 27 inches of mercury. Preferably, the air injectors are positionedabout one and a half meters below a surface of seawater in thereservoir.

The preferred reservoir is generally circular in conform and surroundsan axial water intake and an array of spouts leading to a flashevaporator. The baffle positioned between the water intake and the arrayof spouts for directing water from the intake to the reservoir and fromthe reservoir to the spouts.

A preferred seawater pre-deaerator for open-cycle ocean thermal energyconversion plants includes a vertical riser water intake, a baffle atthe top of the intake directing water from the intake generally radiallyoutward, and a circular water reservoir at the top of the water intakesurrounding the baffle. Water flows from the water intake outward asdirected by the baffle into the water reservoir. An array of spoutsabove the baffle and within the reservoir for releasing water flowingfrom the reservoir into the spouts. Plural air injectors positioned inthe reservoir release air into the reservoir and bubble air upwardthrough the reservoir, removing dissolved gases from seawater in thereservoir.

Preferably, the water intake, reservoir, baffle, spout array andevaporator are axially aligned on a vertical axis and the preferred airinjectors are porous aeration stones arranged in concentric circularpatterns in the reservoir.

This invention provides the method of deaerating seawater which includessupplying seawater to a reservoir, releasing fine seed bubble nuclei gasinto the water in the reservoir, bubbling the gas through the reservoir,withdrawing dissolved gas from the seawater in the reservoir with thebubbles, removing gas from the top of the reservoir and flowingdeaerated seawater from the reservoir.

The preferred method further includes evaporating the deaerated seawaterto form steam, expanding the steam, and condensing the steam by directcontact with additional cold seawater.

The preferred removing step comprises removing the withdrawn gas and thereleased gas by pumping the gas away from a top of the reservoir.

The preferred releasing of seed bubbles comprises conducting pumped gasto a bottom of the reservoir and injecting the pumped gas into thebottom of the reservoir.

Preferably, the injecting gas provides creating bubble nuclei in thebottom of the reservoir through an array of porous aeration stones.

In the preferred method the supplying includes directing water around abaffle outwardly into a reservoir. The flowing includes flowing waterinwardly from the reservoir toward evaporation spouts, and the removingincludes removing gases from a vacuum chamber above the reservoir.

These and other and further objects and features of the invention areapparent in the disclosure which includes the above and ongoingspecification and claims and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred degassificationsystem of the present invention.

FIG. 2 is a cross-sectional plan view of the system shown in FIG. 1.

FIG. 3 is a schematic detail of a preferred seawater degassificationsystem of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a degassification system of the present inventionis generally indicated by the numeral 1. Seawater 3 flows upward througha barometric riser water intake 5 and outward 7 past a baffle 9 into alarge cylindrical reservoir 11. Within the base of reservoir 11 is anarray 13 of individual porous aeration stones 15.

Air injection should be such that small bubbles can be produced. Airinjection done through the porous aeration stones 15, ensures that theair is released in the form of fine seed bubbles. This avoidsappreciable concentration of injected air and the resultant formation oflarger sized air slugs. The streams of these upward flowing smallbubbles are generally indicated by the lines 17. As the bubbles flowupward they expand and form increasing volumes having pressure slightlyhigher than the pressure of the surrounding seawater. Dissolved gasesfrom the seawater are withdrawn into the expanding and upward movingbubbles 17, degassifying the seawater 19 within the reservoir. As theyreach the surface of the seawater in the reservoir the bubbles break upand their gas flows through outlet 39 and collects in a vacuum chamber21 which is evacuated with a vacuum pump 23. An exhaust 25 of the vacuumpump supplies plural air lines 27 through reinjection head 26, leadingto the distribution lines 29 for gas injectors 15. Deaerated seawaterflows under cylindrical baffle 41 and into the volume 31 above centralcore baffle 9 and flows outward through vaporizing spouts 33 into flashevaporator or direct current condenser (if used) 35. The surface of theevaporator may be warmed by sun or seawater or both. Finally, the steam37 formed within the evaporator is flowed to a work producer, such as asteam engine or turbine. A pressure drop across the work producingdevice is established by connecting to its exhaust a condenser which maybe a direct contact seawater condenser. The preferred process isreferred to as an open-cycle process because new water is continuallyused and the used water is returned to the ocean.

As shown in FIG. 3, baffle 9 is replaced by a conical wall 49. Acylindrical baffle 41 at the outlet of reservoir 11 helps to separatethe deaerated water flowing through space 43 under the baffle 41 fromthe rising bubbles 45.

One or more vacuum conduits 47 lead to one or more vacuum pumps. Part ofthe exhaust from the vacuum pumps is reintroduced to the reservoirthrough injectors 15. The rest of the removed gas is returned to thedeaerated water after it has cycled through the work producing systemand as it is discharged back into the ocean.

In one example of operation, the reduced pressure chamber 21 ismaintained at a vacuum of about 27 inches of mercury. The air injectorsare about one and a half meters below the surface of the water in thereservoir which is sufficient to expand the bubble nuclei or seedbubbles as they rise through the water and to withdraw sufficient gaswhile keeping power requirements low. The entire vacuum and airinjection system operates at below atmospheric pressures in a preferredembodiment. There is no separation between water surface in reservoir 11and overlaying low pressure air space which is a low pressure gaschamber 21.

While the invention has been described with reference to specificembodiments, modifications and variations of the invention may beconstructed without departing from the scope of the invention which isdefined in the following claims.

I claim:
 1. A method of deaerating seawater comprising the steps ofsupplying seawater to a reservoir, releasing seed bubbles of gas intothe seawater supplied to the reservoir by means of gas injectors locatedat a bottom of the reservoir, bubbling the gas through the reservoir,withdrawing dissolved gas from the seawater in the reservoir with thebubbles to a top of the reservoir, removing the withdrawn gas from thereservoir by means of a vacuum pump, recycling the removed gas bysupplying the gas from the vacuum pump to the gas injectors, and flowingout deaerated seawater from the reservoir.
 2. The method of claim 1further comprising the step of evaporating the deaerated seawater andcondensing the evaporated seawater by direct contact with additionalcold seawater by a surface condenser.
 3. The method of claim 1 whereinthe removing step comprises removing the withdrawn gas and the releasedgas by pumping the gas away from a top of the reservoir.
 4. The methodof claim 3 wherein the step of releasing seed bubbles comprisesconducting pumped gas to a bottom of the reservoir and injecting thepumped gas into the bottom of the reservoir through the gas injectors.5. The method of claim 4 wherein the step of injecting gas comprisescreating bubble nuclei in the bottom of the reservoir through a porousarray of plural stones.
 6. The method of claim 5 wherein the step ofsupplying further comprises directing water around a baffle outwardlyinto the reservoir and wherein the step of flowing further comprisesflowing water inwardly toward evaporation spouts and wherein the step ofremoving comprises removing gases from a low pressure gas chamber abovethe reservoir.
 7. The method of claim 1 wherein the step of supplyingcomprises directing incoming seawater to the reservoir and wherein thestep of flowing comprises redirecting seawater from the reservoir toprevent direct flowing of the seawater into and out of the reservoir. 8.The method of claim 7 wherein the step of redirecting comprises flowingseawater out of the reservoir beneath a baffle.
 9. The method of claim 6wherein the step of removing further comprises withdrawing gas away fromthe top of the reservoir and reaerating deaerated seawater with thewithdrawn gas by exhausting the withdrawn gas into the deaeratedseawater.